1. Field of the Invention
The present invention relates to refractory ceramic materials and the use of such materials in the manufacture of sheet glass by the fusion process.
2. Technical Background
The fusion process is one of the basic techniques used to produce sheet glass and can produce sheet glass having surfaces with superior flatness and smoothness relative to sheet glass produced by alternative processes, such as for example, the float and slot drawn processes. As a result, the fusion process has found advantageous use in the production of the glass substrates used in the manufacture of light emitting displays, such as liquid crystal displays (LCDs).
The fusion process, specifically, the overflow downdraw fusion process, includes a supply pipe which provides molten glass to a collection trough formed in a refractory body known as an isopipe. During the overflow downdraw fusion process, molten glass passes from the supply pipe to the trough and then overflows the top of the trough on both sides, thus forming two sheets of glass that flow downward and then inward along the outer surfaces of the isopipe. The two sheets meet at the bottom or root of the isopipe, where they fuse together into a single sheet. The single sheet is then fed to drawing equipment that controls the thickness of the sheet by the rate at which the sheet is drawn away from the root. The drawing equipment is located well downstream of the root so that the single sheet has cooled and become rigid before coming into contact with the equipment.
The outer surfaces of the final glass sheet do not contact any part of the outside surface of the isopipe during any part of the process. Rather, these surfaces only see the ambient atmosphere. The inner surfaces of the two half sheets which form the final sheet do contact the isopipe, but those inner surfaces fuse together at the root of the isopipe and are thus buried in the body of the final sheet. In this way, the superior properties of the outer surfaces of the final sheet are achieved.
The dimensional stability of an isopipe during the glass forming process can affect the overall success of the manufacturing process, as well as the properties of the manufactured glass sheet. In the overflow downdraw fusion process, an isopipe can be subjected to temperatures of about 1,000° C. While exposed to these temperatures, an isopipe must support its own weight, the weight of the molten glass contained within the isopipe and overflowing its sides, and at least some tensional force that is transferred back to the isopipe through the fused glass as it is being drawn.
Commercial and market factors require a continuous increase in the size of light emitting displays and thus, the size of sheet glass. Depending on the width of the sheet glass to be produced, an isopipe can have an unsupported length of about 1.5 meters or more.
To withstand these demanding conditions, isopipes have conventionally been manufactured from isostatically pressed blocks of refractory material (hence the name “iso-pipe”). In particular, isostatically pressed zircon refractories have been used to form isopipes for the fusion process. Conventional zircon refractories are comprised of ZrO2 and SiO2, or equivalently ZrSiO4, and sintering additives. Even with such high performance materials, isopipe materials can creep, resulting in dimensional changes which limit their useful life. In particular, isopipes exhibit sag such that the middle of the unsupported length of the pipe drops below the height of its outer supported ends.
Thus, there is a need to address dimensional stability and other shortcomings associated with conventional isopipes and methods for manufacturing sheet glass. These needs and other needs are satisfied by the composition and methods of the present invention.